The present invention relates to gold-supported catalysts wherein the support material is comprised of graphitic carbon nanostructures, preferably layered nanostructures The gold supported catalysts are suitable for use as hydrogenation catalysts
Gold has limited use as a catalytic metal since it has generally been regarded as inert. In fact, it has even been used as a coating for various devices to inhibit the catalytic activity of other metals. Some of the limited catalytic activity of gold include its activity for the low temperature carbon monoxide oxidation and oxidation of propylene when supported on titanium oxide. This was reported by Haruta et al. in Journal of Catalysis, 115, 301 (1989) Also, activated carbon supported gold catalysts are known to be more resistant towards deactivation than the corresponding palladium or platinum on carbon systems for the selective liquid phase oxidation of diols. See Journal of Catalysis, 176, 552 (1998). The advantages of using gold-platinum alloys supported on glassy carbon compared with an analogous pure platinum catalyst for the electro-oxidation of methanol has also been demonstrated. It is also known that the interaction of gold crystallites with supports that are semi-conducting in nature could cause modifications to the metal surface atoms that stimulate catalytic activity. Thus, the chemical and electronic nature of the support appears to play a key role in stimulating the catalytic activity of gold.
The electrical conductive properties of graphite offer advantages when the material is used as a catalyst support medium for small particles. Unfortunately, in its conventional form of flat sheets, graphite has a relatively low surface area (xcx9c0.5 m2/g) and the high fraction of exposed basal plane regions are relatively inert with respect to interaction with other materials. The weak interaction of metals with the basal plane of graphite results in the mobility and eventually sintering of the particles, a phenomenon that reduces the efficiency of the catalyst as a lower surface area is exposed.
Therefore, there remains a need in the art for ways to increase the catalytic activity of gold.
In accordance with the present invention there is provided a gold-containing catalyst, which catalyst is comprised of gold on a nanostructure support, which support is characterized as graphite nanofibers comprised of graphite sheets, which graphite sheets are oriented substantially perpendicular or parallel to the longitudinal axis of the nanofiber and wherein said graphite nanofiber contains exposed surfaces and wherein at least about 95% of said exposed surfaces are comprised of edge sites.
In a preferred embodiment of the present invention, the graphite nanofiber is further characterized as possessing: (i) a surface area from about 0.2 to 3,000 m2/g as determined by N2 adsorption at xe2x88x92196xc2x0 C., (ii) a crystallinity from about 50% to about 100%, and (iii) a distance from about 0.335 nm to about 0.67 nm between the graphite sheets.
In another preferred embodiment, at least a portion of the edges of the graphite nanofiber contain a functional group selected from the group consisting of Cxe2x80x94OH, Cxe2x95x90O, Cxe2x80x94Oxe2x80x94C, and COOH.
In still another embodiment of the present invention, the graphite nanofibers are characterized as having a crystallinity greater than about 90%.
In yet other preferred embodiments, the carbon nanofibers are characterized as having (i) a surface area from about 50 to 800 m2/g; and (ii) a crystallinity from about 95% to 100%.